This invention relates generally to electric arc furnace steel making and specifically to such systems having a ladle metallurgical furnace therein, which systems require decreased energy input per unit of steel produced compared to similar systems. It is particularly directed to making alloy steel at a rate limited only by the maximum melting capacity of the arc furnace. In addition the invention, without modification, is adaptable to nearly every end use found in the steel industry today from continuous casting to unique, one of a kind melts of widely varying compositions in a randomized production sequence.
For example, the invention enables the production of up to four different types of steel (as distinct from grades of steel) in a single electric arc furnace system without slowdown or delay in the processing sequence of heats regardless of the number or randomized order of the different types of steel to be made in a campaign. Thus the system will produce at least non-vacuum arc remelt steel, vacuum arc remelt steel, vacuum oxygen decarburized non-vacuum arc remelt steel and vacuum oxygen decarburized vacuum arc remelt steel.
For some years extending up to about the last decade and a half the vacuum arc degassing system was practiced throughout the world for the production of steel having alloy, gas, grain size and inclusion contents within narrowly defined ranges. In this system steel tapped from an electric arc furnace was thereafter subjected to the combined effects of a low vacuum, a purging gas, and alternating current heating arcs struck between graphite electrodes and the wildly boiling surface of the molten steel while it was subjected to the combined effects of a low vacuum and the purging gas. This system is usually referred to as the vacuum arc degassing system. Millions of tons of steel have been produced by this method and significant tonnage continues to be produced at this date. This method has advantages unachievable by the prior competitive systems including the ability to teem at plus or minus 10° F. at any desired time extending for as long as at least eight hours from furnace tap. Thus a 100 ton ingot could be produced from a system having only one 50 ton arc furnace, and ample time was always available to compensate for planned, or unexpected, downstream delays, thereby avoiding return of a melt to the arc furnace.
However, during normal operations in such systems the production rate or throughput of the system is governed by the processing time in the arc furnace and, in most installations, the processing time for a single heat can be upwards of four to four and one half hours due to the extensive steel making which takes place in the arc furnace; in other words, the steel resides in the arc furnace long after the scrap charge has melted and reached tapping temperature.
With increasing pressures on the steel maker to lower costs and increase throughput using conventional arc furnace technology the lengthy, by comparison, arc furnace steel making technology has had to be abandoned in favor of shorter cycles which achieve the same end result.
For approximately the past 15 years the ladle metallurgical furnace system has begun supplanting traditional arc furnace and vacuum arc degassing steel making technology. In the ladle metallurgical furnace system the arc furnace has been confined to being almost solely a melting unit, with most steel making deferred to downstream operations. For the arc furnace in such a system this has resulted in a much shorter dwell time of the scrap charge in the furnace since raw scrap (and early lime and carbon additions) can be brought to tapping temperature in about two hours, or less, compared to the four to four and one half hours required in conventional arc furnace steel making in the same size furnace. The use of larger electrodes has also contributed to decreased furnace dwell time. In a specific example which will be described in greater detail hereafter, the furnace dwell time from the beginning of charging to the end of tapping will be decreased from four to four and one half hours to two hours or less.